This invention relates to a modulation method, a recording method, a reproducing method, a recording and reproducing apparatus, a recording and reproducing method, and a reproducing apparatus, and particularly to eight-to-ten (8-10) modulation for transforming 8-bit data to a 10-bit code.
In a digital audio tape recorder (DAT) for recording and reproducing audio data into and from a recording medium, such as, a magnetic tape, 8-10 modulation is employed whereby 8-bit data is transformed to a 10-bit code.
Specifically, in the conventional 8-10 modulation, 8-bit data is transformed to a 10-bit code in accordance with a code table in which codes being (0,3;8,10;1) codes and having a so-called codeword digital sum (CDS), that is, a value obtained by subtracting the number of "0s" from the number of "1s" of the code after non return to zero inverted (NRZI) modulation or a digital sum value (DSV) per code, equal to 0, .+-.2, are allocated to 8-bit data, as shown in FIGS. 1 to 7. The code table is hereinafter referred to a DDS2 code table. In FIGS. 1 to 7, a value indicated by Q is the DSV for codes up to the corresponding code, and a value indicated by Q' is the DSV for codes up to the preceding code of the corresponding code. Either one of two codes allocated to one data is employed by a so-called low disparity code (LDC).
DAT has a recording system composed of an 8-10 modulator 61, an NRZI modulator 82 and the like, and a reproducing system composed of an integral equalizer 73, a binary coding circuit 74, an NRZI demodulator 77, an 8-10 demodulator 78 and the like, as shown in FIG. 8. The 8-10 demodulator 61 transforms 8-bit data to a 10-bit code. The NRZI modulator 62 NRZI modulates the code, and supplies the resulting code through a recording amplifier 63 to a so-called rotary head 64. Thus, audio data is recorded onto a magnetic tape 5 by a so-called helical scan.
In reproduction of audio data, on the other hand, the integral equalizer 73 integrally equalizes reproduction signals reproduced from the magnetic tape 5 by a rotary head 71. The binary coding circuit 74 encodes the equalized reproduction signals (hereinafter referred to as the equalized waveform) to a binary code. The NRZI demodulator 77 NRZI demodulates the resulting code to reproduce the 10-bit code. The 8-10 demodulator 78 then transforms this code to 8-bit data. Thus, audio data is reproduced. A phase locked loop (PLL) 75 reproduces a channel clock, and a synchronization detection circuit 76 detects a delimiter of codes, for example.
DAT has also been used as a data recorder for storing data for information processing devices, other than audio data, recently. The data recorder also employs the above-mentioned 8-10 modulator, integral equalizer and binary coding circuit.
Meanwhile, the conventional data recorder employs the above-described integral equalizer. That is, the linear recording density along tracks of the magnetic tape is set to, for example, 61 kbps, which does not cause intersymbol interference in reproduction signals. Therefore, high recording density cannot be realized with the conventional circuit configuration.
Thus, it is conceivable to use a technique of so-called partial response for controlling the quantity of intersymbol interference to increase the linear recording density. Specifically, an equalizer of partial response class 1 may be used instead of the integral equalizer 73, and a Viterbi decoder may be used instead of the binary coding circuit 74.
In the conventional DDS2 code table, codes having a number of consecutive 1's are employed, such as, codes "1111111111", "0111111111" and "1111111110" allocated to data of "11101011", "00101011" and "11111110" in binary expression, as shown in FIGS. 1 to 7. The data of "11101011" is hereinafter referred to as data EB. On the basis of a waveform string obtained by NRZI modulating these codes, recording of data onto the magnetic tape is carried out as inversion of magnetization polarity. Accordingly, when a ternary reproduction signal obtained as differential of magnetization polarity inversion is equalized by partial response class 1 for the code having a number of consecutive 1's, a number of consecutive 0's appear in the equalized waveform.
Two threshold values to discriminate the ternary equalized waveform (a so-called eye pattern) are determined on the basis of envelopes of the equalized waveform so as to correspond to level changes of the reproduction signal. Therefore, a large number of consecutive 0's in the equalized waveform prevent these threshold values from being obtained stably, and thus increase the error rate. In addition, a determination of whether the level is lowered by so-called dropout or by the consecutive 0's cannot be accomplished.
In Viterbi decoding, data cannot be determined when continuous 0's are present in the equalized waveform in so-called maximum likelihood decoding. Stated differently, when a large number of continuous 1's are used as described above, the memory of the Viterbi decoder must be increased. Thus, the circuit scale is enlarged.
Also, in the conventional DDS2 code table, codes "1011001110", "0011001110", "1110001110" and "0110001110" are allocated to data AC and C6, as shown in FIGS. 1 to 7. These codes generate, when used twice, lengths between transition 1T, 4T, 1T, 3T, 1T or 1T, 3T, 1T, 4T, 1T in the waveform string, with T denoting the minimum length between transition. That is, these codes generate a large peak shift, causing the error rate to be higher to that of other codes. In the conventional 8-10 modulation, continuation of identical codes is not taken into account, and the codes of higher error rate are employed.